Your datablock is named playerfastplayer, but you are trying to find the datablock: FastPlayer. Change them to make them match like below:
function SodaCanImage::onDone(%this,%obj,%slot)
{
%obj.unMountImage(%slot);
}
function SodacanImage::onFire(%this,%obj,%slot)
{
%obj.setDatablock(FastPlayer);
}
datablock PlayerData(FastPlayer : PlayerStandardArmor)
{
maxForwardSpeed = 15;
maxBackwardSpeed = 15;
maxSideSpeed = 15;
maxForwardCrouchSpeed = 13;
maxBackwardCrouchSpeed = 13;
maxSideCrouchSpeed = 13;
canJet = 0;
uiName = "FastPlayer";
};